Presently, stainless steels are used for the brake disc of motorcycles but such a disc demands high corrosion resistance, toughness and abrasion resistance as the properties. The abrasion resistance becomes generally higher with increase of the hardness, while the toughness reversely lowers, so that by taking account of this point, the hardness of the brake disc is limited to a range of 30-40 in H.sub.R C.
As stainless steels to be used for this application, use has been made of high carbon martensitic stainless steels, such as 16Cr-0.3C, SUS 420J1 having C of 0.2%. SUS 420J2 having C of 0.3% and for providing the properties for the brake disc materials, heat treatments such as quenching at a temperature of 850.degree.-1,050.degree. C., or such quenching and tempering are carried out.
However, in such prior steels, an amount of carbon in the steels is as high as 0.2-0.3% and the hardness of martensite itself caused in the quenching operation is as high as 50-60 in H.sub.R C and the toughness is poor and quenching cracks are readily caused in the quenching operation.
Accordingly, in order to control the hardness to 30-40 in H.sub.R C only by the quenching operation, the steel should be the mixed structure wherein two phases of martensite and ferrite are mixed in a moderate ratio and the ratio varies depending upon the temperature of heat treatment for quenching and particularly varies in a range of 800.degree.-900.degree. C. and the hardness varies from about 5 to 45 in H.sub.R C. In addition, the hardness is apt to be affected by a slight variation of the components of the steel sheet, so that the very strict and severe control of quality and the control of the conditions of the quenching heat treatment are required in the production.
Furthermore, when the quenching and tempering are carried out, there are caused the problems of the quenching cracks formed in the hardening and the processing trouble. Besides, the tempering is usually effected within a range of 550.degree.-650.degree. C. for controlling H.sub.R C in a range of 30-40, so that a zone of a low Cr concentration is formed around carbide and nitride of Cr precipitated owing to this heat treatment to lower the corrosion resistance.